It is generally known that the use of straight vacuum residue for road asphalt produces a road cover of inferior quality from the standpoint of durability because the paraffin components of the vacuum residues are readily biologically degradable. In time the asphalt cover loses its coherence and becomes brittle.
As a result, various deasphalting processes have been developed in order to obtain a more paraffin-free asphaltic product. The deasphalting process, in general, consists of extracting the waxes from the asphaltenes. The "asphaltenes" are defined as materials which are insoluble in pentane or heptane, but soluble in benzene. The asphaltene materials are also considered to be "heat labile," in that they coke readily at temperatures above about 700.degree. F. Typically the solvents used in "deasphalting" operations to separate the waxes from the asphaltenes are propane and mixtures of propane, butane and pentane.
It has, however, recently been determined that the various groups of compounds making up these mixtures called "asphaltenes" still contain up to 50% microcrystalline waxes. This discovery came to light from a new analytical method designed to analyze recovered asphaltenes and reference may be made to: A.P.I. Research Project 60, Report No. 13, "Characterization of the Heavy Ends of Petroleum," July 1, 1972, to June 30, 1973. In essence, it appears that the high percentage (up to 50%) of microcrystalline waxes remains because they are soluble in heptane at its boiling point of about 100.degree. C. while only soluble to a small extent at ambient temperatures. In the standard asphaltene test, where there is testing with pentane or heptane at ambient temperatures, the microcrystalline waxes are practically insoluble in the solvent and tend to simulate asphaltenes and will to some degree co-precipitate with them.
By way of comparison, the new method of asphaltene component analysis demonstrates that the so-called asphaltenes, as determined by insolubility in heptane, actually consist of about 50% microcrystalline waxes, on the average. This, in turn, means that the asphaltene percentages previously published in the literature for crude oil, as to atmospheric distillation residues and vacuum distillation residues, should be cut by about fifty percent.
Although there are less asphaltenes, i.e., "true" asphaltenes, in crude oils than heretofore reported, it is to be emphasized that the asphaltenes recovered in an improved separation procedures will be less adulterated; and, as a result, not subjected to the rapid deterioration of the mixes which contain up to 50% waxes. In addition, it is to be pointed out that a better separation of the waxes provides a greater field of material suitable as a valuable source of distillate. Asphaltenes cannot be cracked to distillate. Where they are left in distillate cuts, they mostly form gas and coke, causing difficulties in the refining operations. Actually, as heretofore noted, the presence of the heat labile materials prevents the total desulfurization of the residual fuels in the present-day catalytic hydrodesulfurization units. The commercial desulfurization of residual fuels containing asphaltenes achieves a reduction of the sulfur content to no better than 0.2% to 0.3% while the same residual fuel can be desulfurized to less than 0.01% sulfur provided the true asphaltenes are removed. Furthermore, catalyst life can be extended from one year to about eight years.